The inventive subject matter relates to power supply apparatus and methods and, more particularly, to AC power supply apparatus and methods.
UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations to provide backup power to maintain operation in event of failure of the primary utility supply. UPS systems commonly use an inverter that generates an AC output from a DC power source, such as a battery. In an “off-line” or “standby” UPS, for example, a load may be switched from an AC utility source to an inverter powered by a battery when the utility source fails. This may result in a momentary outage to the load.
A load served by such UPS may have characteristics that place relatively large initial current demands on the UPS when transferring between normal and backup modes in responsive to an outage. For example, computers and similar electronic equipment may include power supplies with power factor correction (PFC) circuitry. When using an off-line UPS to serve such a load, a loss in a utility source may cause an outage on the order of 4-5 milliseconds before the inverter is activated to provide power to the load from a battery. When the inverter of the UPS activated to serve the load, PFC circuitry in the load's power supply may momentarily significantly increase current demand in relation to the device's normal steady-state demand as storage capacitors in the PFC circuitry are recharged. This high current demand may last one or more cycles of the AC voltage.
In a three-phase off-line UPS, this current demand may cause one or more of the phases to exhibit a diminished voltage for an extended period due current limiting by the inverter, which may cause under-voltage related disruptions and/or malfunctions. Conventional approaches for dealing with such issues include derating the UPS so that that lead under these types of transient conditions does not exceed the peak current capability of the UPS.